This invention relates to a light receptor device for cameras in which a pair of photoelectric cells of substantially the same characteristics are directly connected such that their respective electromotive forces counteract each other to thereby improve the illuminance-output characterisitc.
Heretofore, Se photocells, Cds photoconductors and silicon photocells have been used as photoelectric converting elements for various applications according to their characteristics. Among others, silicon photocells, owing to their quick response and high reliability, have recently come to be used popularly as light receptor means for punch cards, tape reading devices or pattern discriminators of electronic computers or for automatic exposure regulators of cameras. Application of the silicon photocells as camera elements proves to be particularly advantageous because quick response and reliability are important factors for obtaining good photograph quality.
Generally, when a silicon photocell is used as a photodetector in a normal condition, an arrangement is usually made such that a proportional relation is maintained between illuminance and output (voltage or current), and this can be attained by suitably selecting the external load connected to the photocell and using the "straight" portion of the characteristic graph.
However, a problem arises in regard to using such photocell as a light receptor for the automatic exposure regulator in a camera under normal conditions. That is, since the illuminance-output characteristic of the photocell maintains the proportional relation even in the high illuminance range, the photocell output in the high illuminance section in the logarithmic representation of characteristics is extremely elevated, and this necessarily causes an increased angle of deflection of the ammeter pointer or such which is operated by such output of the photocell.
According to the well-known techniques of the prior art, deflection of the ammeter pointer is utilized as a driving power source of the stop member, and a tapering aperture which gradually changes in opening is formed in the stop member. Thus, when illuminance is high, the stop member is greatly displaced and only an extremely small portion of the aperture is formed in the exposure opening.
In such a case, if the photoelectric output characteristic holds a proportional relation even in the high illuminance range, the stop member is forced to make an unnecessarily large displacement, so that it is required to form the continuous tapered aperture which is extremely fine and elongated. This poses manufacturing difficulties.
In addition, an excessive amount of movement of the stop member is an undesirable factor in the construction of the camera. Further, the characteristics of the individual photocells inevitably vary from cell to cell, and also the proportional portion of the characteristic graph of photocells tends to be greatly affected by temperature. Thus, improvements have been required in photocells used for automatic exposure adjustment in cameras.